Water Rockets

How to Make a Water Rocket:

Water Rockets are made from 2-liter or 20 oz. soda bottles and can be as simple as an empty bottle or embellished to include fins and even a parachute. At the cuborees we will launch the water rockets that the boys have made and brought with them.

Water Rockets can go extremely high depending on the amount of water in the chamber, the pressure, the weight and the aerodynamic stability of the rocket.

Building a Rocket

Start with a 2-liter or 20 oz. soda bottle. (Don't use a water bottle; soda bottles are built to withstand greater pressure.) Take off the wrapper. It's very important that you do not cut the plastic of the bottle at any time during construction. Be extremely careful when taking off the wrapper.

You can decorate your rocket any way you would like. Paints must

be flexible, as the bottle will expand and contract quite a bit during

pressurizaton and launch. Sharpie markers have been found to

work well. You could also decorate a piece of paper and then

tape the paper to the rocket with a durable tape.

You may stop at this point if you would like. However, an empty bottle is not aerodynamically stable. Once the water is gone, the bottle will cease to fly in a straight line and begin to tumble. It will still work, but not go as high as it could.

Fins can be added to make a Rocket aerodynamically stable. A stable rocket continues to fly straight and gain altitude long after the water is gone.

Construction Techniques

Fin Material

Some have found Foam Core (or Foam Board) to work very well. It's the material used to provide backing to pictures before framing. It's light weight, easy to cut, rigid and inexpensive. You can find it at most art supply stores. You can buy it from Wal-mart or craft stores.

Fin Patterns

Others use plastic cut from bottles to make fins, in any case, make sure all fins have at least an inch of clearing around the sides of the neck of the lower bottle.

Adhesion Techniques

Clear packing or electrical tape works well as an attachment method, but some find it difficult to work around the curved parts of the bottle with packing tape.

Several glues have also been found to work very well: PL Premium Construction adhesive (a polyurethane based glue), or E6000. Both are available from hardware stores. Some problems with these products are that the glue takes time to dry but it's very strong.

For some, the easiest method is Hot Glue. It's important to use a Low Temp glue and gun. A high temp glue gun can melt the bottle. You may find that even a low temp glue gun will warp the plastic of the bottle, but don't worry, it won't weaken the plastic.

One method of attaching the fins is outlined below:

(this is certainly not the only way)

Some use a sharpie marker to mark the location of the fins. First take a plain piece of paper. Cut the paper lengthwise and tape the paper end to end to make a strip long enough to make it around the bottle. Make sure the two pieces are straight. Wrap the paper around the bottle and draw a line on the paper where the ends meet. Take the paper off the bottle and lay it on a table. The distance from one end of the paper to the line where it met will be about 13.5" (if not, make sure that the lines are spaced 1/3 of the way around between each) Draw two points on the paper tape, one 4.5" from the end and then next 9". Wrap the paper around the bottle again and tape it together. Put three marks on the bottle. One where the paper is taped together, the second at the 4.5" mark, and the third at the 9" mark. Take the paper off the bottle. Place the bottle against a door jam, or some other corner to act as a straight edge and use it to extend the marks along the bottle.

Then, mark the location for the top of the fins by holding one fin against the rocket until it is where you want it and then mark the bottle at the top of the fin. Holding a pen in one hand and rotating the rocket that is resting in a groove with the other, draw a mark around the body of the rocket.

Fill the bottle with cold water. With fine sandpaper lightly rough up the area to be glued. Apply the glue to the fin material and then attach the fin to the body of the rocket. This technique works great because the glue dries very fast, the weight of the water provides some stiffness to the bottle, and there is no warping (again, slight warping of the bottle with low temp glue will not affect the strength of the bottle).

Glue the fins to the rocket trying to keep them as straight as possible.

Parachutes

Parachute deployment in a water rocket is a significant challenge. There are pages and pages written on different techniques. If you're familiar with Estes rockets, an Estes rocket engine has a small charge at the front of the engine that fires up the inside of the rocket to blow the parachute out at the appropriate time. There is no such thing with a water rocket. Here's the easiest technique some have found. Make a parachute by cutting a circle between 12"-18" in diameter from a plastic garbage bag. Secure six pieces of string equidistant around the circumference with tape. Gather the other end of the strings together so that they are even and secure them together with a couple of good knots. Use the hot glue gun to glue this knot to the top of the rocket. (You may also use an Estes model rocket parachute that you can purchase separately from the hobby store.) The chute is then folded and placed on top of the rocket. A cut-off portion of a second bottle is loosely placed on top. The idea is that the pressure from the air pushing down on the rocket as it travels upward keeps the cover attached, but when the rocket reaches its peak and turns over the top separates from the rocket and the parachute unfolds.

That's how it's supposed to work. On my first try I cut the cover so it was too large. It created a seal on the top of the rocket and the parachute never deployed. In my second try the top was too small and came off as the rocket took off. Give it a try if you want and check out the web for other very creative ideas people have come up with for deploying a parachute. Parachutes are not required.

Other Construction Ideas

Many have made rockets with extensions to make them longer and more stable. A second bottle is cut and taped to the top of the first bottle. The main bottle below that holds the water and pressure remains uncut. This is done for esthetics and to make the bottle more stable because it moves the center of gravity forward (more on this later). Feel free to do this but always leave the bottle on the bottom, the one that will hold the water and the pressure uncut.

Aerodynamics

To be stable the center of gravity of the rocket (where the rocket balances) must be in font of the center of pressure (if you put a rocket in a wind tunnel sideways and held it at this point the rocket would not turn. The force of the air pushing against the front of the rocket is the same as the force pushing against the back of the rocket). To move the center of pressure backwards you make larger fins or move the fins toward the tail, or even behind, the rocket. To move the center of gravity forward you make the rocket longer or add weight to the nose. Some use a small piece of clay stuck to the front of the rocket.

Testing a Rocket for Stability

Find the center of gravity by balancing the rocket. Tie one end of a long piece of string (6-10') around the rocket and tape it at this point. Swing the rocket at the end of the string around your head and notice its flight characteristics. If it flies nose first all the way around then it's stable.

Launch Sequence

The Rocket is filled a third of the way with water. The rocket is then slid onto the launch tube. The nozzle of the rocket makes a seal around an O-ring at the base of the launch tube. At that point 2 prongs, run through eyelets, fit over the lip of the bottle. The rocket is then pressurized via an air compressor or pump. As the pressure increases, everyone stands back. A quick pull on the launch cord pulls the prongs out of the eyelets which lets the rocket explode upwards.... BANG!

Submitted by Don Bottelsen in April, 2005 (edited April 2007 for use on this site)